Re: [Qemu-devel] [PATCH] block: limited request size in write zeroes unsupported path

[Denis V. Lunev]

On 05/01/15 14:29, Peter Lieven wrote:
> If bs->bl.max_write_zeroes is large and we end up in the unsupported
> path we might allocate a lot of memory for the iovector and/or even
> generate an oversized requests.
>
> Fix this by limiting the request by the minimum of the reported
> maximum transfer size or 16MB (32768 sectors).
>
> Reported-by: Denis V. Lunev <address@hidden>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>   block.c |    5 ++++-
>   1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/block.c b/block.c
> index a612594..8009478 100644
> --- a/block.c
> +++ b/block.c
> @@ -3203,6 +3203,9 @@ static int coroutine_fn 
> bdrv_co_do_write_zeroes(BlockDriverState *bs,
>
>           if (ret == -ENOTSUP) {
>               /* Fall back to bounce buffer if write zeroes is unsupported */
> +            int max_xfer_len = MIN_NON_ZERO(bs->bl.max_transfer_length,
> +                                            MAX_WRITE_ZEROES_DEFAULT);
> +            num = MIN(num, max_xfer_len);
>               iov.iov_len = num * BDRV_SECTOR_SIZE;
>               if (iov.iov_base == NULL) {
>                   iov.iov_base = qemu_try_blockalign(bs, num * 
> BDRV_SECTOR_SIZE);
> @@ -3219,7 +3222,7 @@ static int coroutine_fn 
> bdrv_co_do_write_zeroes(BlockDriverState *bs,
>               /* Keep bounce buffer around if it is big enough for all
>                * all future requests.
>                */
> -            if (num < max_write_zeroes) {
> +            if (num < max_xfer_len) {
>                   qemu_vfree(iov.iov_base);
>                   iov.iov_base = NULL;
>               }
Reviewed-by: Denis V. Lunev <address@hidden>

Though pls consider my patch v3, it avoids allocation of 16 Mb here and
uses only 1 Mb of memory.